Electric discharge device



Nov. 8, 1938.

D. GABOR 2,136,292

ELECTRIC DISCHARGE DEVICE Filed April 29, 1936 2 Sheets-Sheet 1 f mw...

Nov. s, 193s. SABOR 2,136,292

ELECTRIC DISCHARGE DEVICE Filed April 29, 1936 2 Sheets-Sheet 2 1:' 1 g' .l El .l

0?/05- 7.o gal/.is

o 50 ibo (v0/fs 1.50

Patentedl Nov. 8, 1938 UNITED STATES PATENT OFFICE Application'April 29, 1936, Serial No. 77,027 In Great Britain April 29, 1935 11 Claims.

This invention relates to improvements in electric discharge devices of the kind described in the co-pending application U. S. Serial No. 747,593, now Patent No. 2,068,287, in which strong electron beams are produced by means of a plasmaillter and shot into a space containing rariiled gases or vapors. The plasmafllter is a perforated body, which is either constructed entirely of insulating materials, or which consists of a carrying structure of conducting materials, covered at least on the parts of its surface exposed to the bombardment of positive ions with insulators.

The openings of the plasmafllter are so restricted as to produce a space-charge limited, positive characteristic.

In one pattern described in the co-pending application the plasmalter separates two discharge spaces, to' be called inner and outer discharge spaces. The outer space is the space into which the electron beams are projected. The discharge in the inner" space is serving as a gaseousor plasma-cathode for the discharge produced in the openings of the filter.

I have found that under certain conditions a new kind of discharge takes place in the inner space. The object of this invention is to specify the means by which the new phenomenon can be produced and utilized. The new discharge is characterized by that in the inner discharge space, between the electron source and the filter, no appreciable number of ions are produced and hardly any light. I call it therefore dark discharge.

In the accompanying drawings I have shown for purposes of illustration some of the various forms in which my invention may be embodied.

Figs. 1 and 2 are cross section and longitudinal section of a simple device for demonstrating the principles of the invention. Fig. 3 is a section of 40 a particular pattern of the plasmalter. Fig. 4

is an electron beam source for direct current.

Fig. 5 is a particular pattern of a lamp according to the invention, for alternating current. Fig.

6 is another pattern of an electron beam source.

Fig. 7 is a diagram of voltages and currents, ob-

tained. with the said source at different dimensions of said source. Fig. 8 is another pattern of a direct current beam source. Fig. 9 is a longitudlnal section of a discharge device according to the invention, the characteristic of which can be electrostatically controlled. Fig. 10 shows a set of current-voltage characteristics obtained by said control. Fig. 11 is a direct current electron beam source, fitted with electrostatic control, in a circuit which produces a modiiied (ci. 17e- 126) characteristic. Fig. 12 is an arrangement for magnetic control of the discharge. Fig. 13 shows a set oi characteristics obtained by said control. Fig. 14 is a further modification of a discharge device according to the invention, in which the dark discharge is bounded by two gridlike structures. Fig. 15 is a direct current electron beam source, whereas Figs. 16, 17 and 18 are different sections of an alternating current source, embodying said modification. Fig. 19 is a discharge device with cold cathode according to the invention, embodying electrostatical control.

In Figs. 1 and 2 a simple device is shown in which the new phenomenon can be produced and demonstrated. In Fig. 1, which is a cross section of the device, I is a cylindrical cathode of the indirectly heated type, 2 is the plasmafilter. It is also cylindrical and may be made of metal gauze, coated at least on its outer side and preferably also in the walls of the openings with insulating materials. Such a coating can be made e. g. by spraying the wire gauze from the outside with an emulsion of insulating materials and hardening it by subsequent firing. In Fig. 2, which is a longitudinal section of the same device, 4 is the cathode, 5 the heating spiral or filament. The plasmalter forms the mantle of a cylinder 6, the ends of which are closed by plates l and 8, one of which is made preferably of transparent material, e. g., of mica, in order to make possible the observation of the discharge phenomena in the inner space. The end plates are fitted with insulating bushings 9 and H0 for the leads of the cathode and of the heating lament. Il is the anode. The whole device is understood to be enclosed in a vessel containing rareed gases or vapors.

For starting the discharge, voltage is applied between cathode and anode. The carrying structure of the plasmalter can remain floating electrically, but it can be also connected with the cathode or--for making the starting easieracross a high resistance with the anode. When the cathode is heated up to a sufciently high temperature, a strongly luminous phenomenon is produced in the outer space, between lter and anode, by the electron beams produced in the nlter, whereas the inner discharge space remains dark, although currents of several amperes might ow through it.

EampZe.-The plasmaiilter is a wire gauze with 40 wires of 0.006" diameter to the inch. It forms a cylinder with 2 cms. length and 2 cms. diameter. It is coated at the outside with a thin layer of ceramic insulating materials, hardened by firing. The dark discharge can be produced also with a bare gauze. in this case however the gauge will be soon destroyed by the bombardment of the positive ions. The cathode may have a diameter of 0.3 cm. and a length of 1.5 cms., giving an area of about 1.5 cm2. and may be coated with barium oxide. At a 1suillciently strong heating of the cathode, the dark discharge produced can carry, e. g., 1 amp. at a total voltage drop of about 25 volts in neon up to about 1 mm. pressure and at about 15 volts in argon up to about 0.2 mm. pressure. These are approximately the minimum voltages at which the dark discharge can be produced in these gases. By means of plasmalter with iine'r or deeper holes and by using the means to be specified later, the dark discharge can be produced with any voltage exceeding these minimum voltages, up to several hundred volts and with currents ranging from a few milliamperes up to several amperes.

At higher pressures or at lower cathode temperatures a bright discharge is produced in the inner space, i. e., a normal cathodic glow. with a drop of, e. g., 17 volts in neon.

Probe measurements have shown, that the whole voltage drop in the inner space amounts in the dark discharge only to 1-2 volts, or might be even as low as a fraction of a volt. The electron temperature is very low, of the order of 0.5 volt or even less. These small electron energies are not sufficient for producing an appreciable number of ions in the inner space. The space charge of the electrons is however completely neutralized in the inner space by ions which are supplied to the inner space through the holes of the filter from the outer discharge space. Ions and electrons are moving in opposite directions through the dark space-shaded in the figuresas indicated diagrammatically by the arrow a and the dotted arrow b. In reality their paths are curved and very considerably prolonged by multiple collisions.

The following explanations may be helpful for an understanding of the mechanism of the dark disch-arge and its novel features: It is known, that the cathodic sheath in glowand arc-discharges has the function of supplying ions to the cathode. In the case of cold cathodes, the ions have to liberate the electrons in the cathode material. On hot cathodes the ions have to neutralize the space charge of the electrons before the cathode. It is known, that the supply of ions can not be less than a certain minimum proportion of the electron current, equal to the square root of the ratio of electronic to ionic mass. This minimum ionic current is, e. g., 1/207 of the electron current in the case of neon, 1/ 605 in mercury. This relation obtains for currents which the cathode can emit without an electric field on its surface. It is called zero-field emission". In the case of tungsten and similar cathodes this means currents less than the saturation current. In the case of oxide cathodes and other cathodes with a small work function there is however no saturation proper, as an electric field can drag a fur'ther current out of the cathode. This is called field-emission. An electric eld at the cathode surface can be produced by an ion supply exceeding the above mentioned minimum supply. Oxide cathodes can therefore easily supply more than ten times their zero-field emission current, as the cathodic glow is able to supply a high excess of irons.

We can characterize therefore the dark discharge by that the source of ions, which in other discharges has its seat `in the cathodic glow, immediately surrounding the cathode, is detached from the cathode, removed from it to a certnln distance, and attached to the plasmalter. 'Ihis implies the possibility of several new and valuable technical applications, which obtains so long as it is possible to secure the stability of the dark discharge and make it safe against a changing over into a normal, bright discharge.

I have found that the formation ofv an ironsupplying cathodic glow can be prevented lby making the cross-dimensions of the openings of the filter smaller than a certain critical length d, which can be calculated from the following formula:

In this formula s is the maximum number of ions or secondary electrons, which can be produced on a path of 1 cm. by an electron of an energy equal or less than the operating voltage of the device, in the filling gas or vapor, at the actual pressure obtaining in the filter holes. m is the mass of the electron, M the mass of an ion. The depth of the holes is chosen preferably equal to d or not much less than d.

Example.-The operating voltage be 120 volts, the filling gas neon at 0.4 mm. pressure. 120 volts are less than the voltage at which s becomes a maximum, therefore the value of .s corresponding to 120 volts must be taken. This is 3.0/cm. for 1 mm. pressure, i. e., 1.2/cm. for 0.4 mm. pressure. With this gives d=0.016 cm. or 0.16 mm.

I have found furthermore, that plasmafllters the inner face of which is conducting and connected with the cathode, or with a potential negative against the cathode, give increased safety against the dark discharge changing over into a bright discharge. An example of such a filter, made of metal gauze, the inner side oi which is left bare, has been described in connection with Figures 1 and 2. Another advantageous construction is shown in Fig. 3. The carrying structure is here a thin metal sheet I2, which has a great number of conical impressions Il, pierced at their bottom by holes Il, which are pointing towards the outer discharge space. On

the outside the sheet is covered with a layer IB.

of insulating material. The same filter can be also made out of a moulded ceramic body, coated by spraying or in some other way with conducting materials at the inside. The shape of -the openings as shown in Fig. 3 gives both a high stability of the dark discharge and safety against disintegration of the metallic sheet or layer by positive ions.

I have found, that the distance of the cathode from the filter is preferably chosen equal to several, e. g., 5-25 mean free paths of molecules. If the distance is too small, e. g., less than ve mean free paths, a considerable proportion of the ions reaches the cathode with the full energy corresponding to the total voltage drop in the lter, or not much less, and the cathode is rapidly disintegrated. If however the distance is, e. g., equal to ten mean free paths, the number of ions which have not lost nearly all their energy by collisions becomes exceedingly small. This follows from that ions and molecules have very nearly the same masses, therefore the energy of the ions is halved in the average at every collision. The number of collisions suffered by an tron beam sources and the positive column are ion increases on the other hand strongly with the distance, approximately with the square of it. Therefore a cathode at a distance of, e. g., 10 mean free paths is safe from disintegration, even at voltages which exceed several times the disintegration voltage of the cathode. Too great distances do not offer further advantages, but increase the striking voltage.

According to the invention the cathode is constructed with an emitting surface so large as to be able to emit the operating current at zero field, at a temperature compatible with a long life. As explained above, the operating temperature must be higher than at which the cathode could emit the same operating current in the presence of a cathodic sheath. By this measure I make the characteristic of the device spacecharge limited and independent of the emissivity of the cathode. This can be shown as follows: As I have explained above, a cathode emitting at zero field requires always the same number of ions for every electron emitted, e. g., 1/01 in neon. In a given geometrical arrangement of the filter and of the cathode and at a given gas pressure, for every ion reaching the cathode a certain larger number of lons has to be dragged through the filter holes. Therefore the ratio of ionic and electronic current in the filter is also constant. This determines the volt-ampere characteristic completely, independently of the emissivity of the cathode, if it is only large enough. The current up to which discharge devices according to the invention may be operated depends mainly on the free area of the plasmafilter, i. e., on the total area of its openings, and is-other things being equal-proportional to it. The averaged current density, i. e., the operating current, (in the case of alternating current, the peak operating current) divided by the said total area may exceed 0.25 ampere/cm2 in devices according to the invention.

Fig. 4 is an electron beam source for direct current, for a lamp according to the application U. S. Serial No. 747,593. The large cathode is made of two nickel discs I6 and Il, of which I6 is coated with substances of low work function, e. g., barium oxide. The two discs are joined on their rim I8 and enclose the heating spiral i9, which may be of tungsten, insulated with alumina. The outer end of I9 is connected with the cathode, the inner end4 20 is passed through the insulating tube 2l. The cathode is mounted by means of thin strips 22 on the base plate 23. 'I'his plate closes the bottom end of the insulating tube or casing 24, the other end of which is closed by the plasmafllter 25. The sleeve 26 is the anode.

Electron beam sources of this type can be used advantageously in light sourcesy in which only a part of the light is produced by the electron beams, and part of it in a positive column. An example of a lamp of this kind,for alternating current, is shown in lFig. 5. The energy of the electron beams is here transformed in Athe wider. parts 21, 28 of the vessel, whereas 30 contains the positive column. The electron beam lsources are" I current always ,a certain ionic current.

ion-collector intercepts, e. g., 90% of the ions,

dimensioned in such a way, that the positive characteristic of the beam sources overcompensates the negative characteristic of the column, and a positive resulting characteristic is produced. Such lamps can be operated on the mains of lighting systems, without resistance or choke.

A further advantage of this device is, that the pressure can be chosen considerably lower than in ordinary lighting tubes. Whereas in arc discharges at low pressures the cathodes are strongly sputtered, because the cathode drop assumes rather high values, the drop at the cathode in the dark discharge is always only of the order of one volt. The best luminous efficiencies are obtained, e. g., in n'eon at pressures ,of only 0.3-0.5'mm.,. commercial lighting tubes had however. hitherto to'be filled with more than 1 mm. Lamps '-ofr'the' kind as shown in Fig. 5 can be made therefore with higher elciencies than hitherto obtained in commercial lamps.

I have pointed out, that it is a characteristic feature of the dark discharge, that no ions are produced in it. In the following examples l.' utllize this property for determining and controlling the characteristics of devices operating with the dark discharge, as follows: A part of the ions moving from the filter towards the cathode is intercepted by means of parts, to be called 'ioncollectors, these partshaving surfaces with po- '-.tentia1s negative to the plasma of the dark fitted with an insulated diaphragm 32, with the` diameter d, which is serving as ion-collector. The effect of varying diameter d on the characteristics, which I have observed, is shown in Fig. 7.

It is a very surprising effect, that a wide diaphragm, which would not hamper other gas discharges appreciably, can raise the voltage for a given current by 100 volts or more. This effect can be explained as follows:

The cathode requires for a certain electron If the this means that ten times more ions must be supplied by the discharge in and near the holes of the plasmafilterl 'Ihese additional ions, which must be dragged through the holes of the filter, produce however a strong positive space charge in and before the outer entrances of the filter holes and this space charge determines the characteristic. All characteristics obtained in this way are independent of the cathode heating, if it is only strong enough for supplying the operating current vat zero field.

Fig. 8 is another pattern of an electron beam source, fitted with an ion-collector. The cathode is made of two nickel shells, 33 and 34, of which 33 is coated with substances of low work function. Both shells are enclosing an annular space, which contains the heating filament 35. The diaphragm 36 is serving as an ion-collector.

It is made preferably of insulating substances or of metal coated with such. This construction offers an increased safety against disintegration of the cathode. In the plane of the diaphragm Il, at the distance D from the filter, there are still a considerable number of ions with energies sufficient for disintegrating metals or oxide coated cathodes. .The cathode 3l can be hit however only by ions which have suffered several collisions and have lost hereby the best part of their energy. 'Ihe characteristic of ,this device can be varied, without changing the parts, by shifting the cathode and the diaphragm together with respect to the filter. An increasing distance D has the same eii'ect as a decreasing diameter d in Fig. 1.

I have found, that ion collectors made of thin wires forming a grid or a gauze have a particular effect and special applications. An example is shown in Fig. 9. 'I'he arrangement is the same as in Figures l and 2, but the device is fitted with a grid 31, made of thin wires. with a special lead Il, which is led through the bushing ll. Whereas the ion-collecting action of the diaphragms as described above is nearly independent of their potential, provided that it is negative, a grid or gauze made of thin wires collects more and more ions with increasingly negative potential. The explanation of this is, that the thin wire surrounds itself with an ionic sheath, the outer surface of which acts as effective ion-collecting area and increases approximately proportionally with the voltage. I call the voltage between grid and cathode Eg. Increasingly negative Eg has therefore the same edect as decreasing d in Fig. 7. Fig. l shows the characteristics: anode current plotted against anode voltage at different grid potentials E', which I have obtained experimentally with a device as shown in Fig. 9. The grid was a spiral made of 0.1mm. wire, with a pitch of 1 mm., at a distance of l mm. from the cathode. The filling gas wasneon, at a pressure of 0.4 mm. The great steepness of the characteristics is partly a consequence of the wide meshes of the filter, but is also due to the fact that only the outside of the filter was coated with insulating material. It is seen that if E; is decreased by l volt. the characteristic is shifted by about 5 volts towards higher anode voltages.

I call this device dark discharge amplifier. Its mechanism is fundamentally different from that of the vacuum electron tubes and of the known gas filled amplifiers. In all known amplifiers the electron current is controlled by the electrostatic repulsion of the grid. In the dark discharge amplifier however the electron current is controlled indirectly, by intercepting ions. By this the discharge is forced to send more ions lthrough the holes of the filter and to increase the space charge of the ions in and before the holes. causing a modification of the characteristic.

Fig. l1 shows another application of a grid as an ion-collector. In an electron beam source similar to that in Fig. 4 the grid 40 is connected with the negative terminal of the current supply, whereas the cathode is connected across a resistance 4I with a few ohms. In this arrangement the grid voltage becomes with increasing cathode current increasingly negative. This results in a more positive characteristic. In Fig. l0 C is a characteristic obtained by biassng the grid on a resistance of 8 ohms. The slope of C corresponds to a resistance of 51 ohms. The

alsace:

energy losses however correspond only to 8 ohms. and even these can be made useful by embodying the resistance in the cathode heater.

I have found furthermore that the dark discharge can be controlled not only electrostatically but also magnetically, by fields which are too weak to influence other gaseous discharges appreciably.

Fig. 12 is an arrangement for magnetic control. The electron beam source is of the same construction as the one in Fig. 8. Coaxially with this is situated a coil 42, energized by the controlling current. Fig. 13 shows the effect of the field on the characteristics. It is seen that a neld of only gauss reduces the currents at a given voltage to about one third. 'I'his effect can be explained as follows: The electrons are moving in the neighbourhood of the cathode at a right angle to the field or a component of it. By this a part of them is forced to return to the cathode.A This however would not in itself modify the characteristic, as according to the invention the emissivity of the cathode is in excess of the operating current. An electron which returns to the cathode takes however its share in ions both in leaving and in reentering the thin sheath at the cathode, without contributing to the current. The characteristics are therefore modified also in the case of magnetic control by the space charges of the additional ionic supply at the filter holes.

Several applications of this new effect are obvious for those skilled in the art.

In all the previous examples hot cathodes have been used as electron sources. I have found however that it is also possible to use gaseousor plasma-cathodes as sources of slow electrons. Fig. 14 shows diagrammatically a device for produclng the dark discharge with a plasma-cathode. 43 is a discharge vessel, containing gases or vapours or a mixture of such at low pressure. 44 is a hot cathode, which may be of any type used in arc discharge devices. 45 is a plasma filter, the conducting structure of which is connected with the conducting casing 46, the other end of which is closed by a fine-meshed metal gauze or similar structure 41. 48 is the anode.

The discharge is started by connecting the casing 48 and the gauze 4l across a high re-l sistance 49 and the switch- 50 permanently or temporarily with the anode 48. After striking a normal cathodic glow appears at the cathode, which fills the space 5i wholly or partly and ionizes it up to the gauze 41. The space 52, shown shaded in the drawings, remains dark. The mechanism of the discharge in this space is the same as in the previous examples, but now a plasma is serving as an indestructible, gaseous cathode. The electrons moving through the dark discharge space are. accelerated in the plasma filter 45 and shot into the lighting space 53.

It is a very surprising effect in this new discharge, that the metal gauze 4l and the plasmafilter-45, which might have nearly the same dlmensions, are performing so diii'erent functions. Whereas in 41 a voltage drop of the order of only one volt is produced, the drop in 45 might be as high as several hundred volts. This again is a consequence of the main feature of the dark discharge, that no ions are produced in it and all ions are supplied to it from the filter. In the holes of the gauze the ions and electrons balance each other optimally. As however ions are necessarily lost between filter and gauze, the ionic current in the filter is higher than in the gauze-and might exceed it many times,so that a strong positive space charge is created before the entrances of the filter holes.

At higher pressures, or if the meshes of the lter or of the gauze are too wide, a normal cathodic glow is produced at 41. As now the discharge in the lighting space need not supply any more an excess of ions, the drop in the fllter becomes also small. Unless its openings have a considerable depth, this will be little more than the normal cathodic drop. In this case the voltage drop in the whole device can be little more than the sum of three cathodic drops. It is therefore of rst importance to prevent a bright discharge in the space 52. safeguarded if the above given rules are observed and if the meshes of the gauze are kept suiiiciently fine, preferably ner than the lter.

This device has the advantage, that the virtual cathode, as represented by the openings of the gauze 41, is indestructible and can bear current densities of any magnitude. The actual cathode 44 is operating under the same conditions as arc cathodes and is abundantly supplied with positive ions by its cathodic sheath. Therefore the characteristic becomes independent of the cathode heating, even if the cathode can emit only a small fraction of the operating current at zero field.

The following gures of the accompanying drawings show examples of devices with plasmacathodes, serving as electron sources for th"e dark discharge. Fig. 15 is an electron beam source for direct current. 54 is the plasmalter, the carrying structure of which is connected with the conducting casing 55. This is tted with the diaphragm 56, the opening of which is covered with a iine-meshed wire gauze 51. The base plate 58 is fastened at the insulating tube 59, through which the cathode 60 projects into the lowerdischarge space 6|. The cathode is an indirectly heated cathode with the heating filament 62. It is however understood that also directly heated or self-heating cathodes and also cold cathodes, e. g., of the pool type can be used in the devices according to the invention.

'I'he end of the heating spiral is led through the insulating tube 63 and connected with the anodic lead 64. The cathode tube 60 is led through 59 and fixed by means of a sleeve 65 on the cathodic lead 66. Both leads are sealed into the stem 61. The anode 68 forms a sleeve around the insulating case 69, which is protecting 55 against disintegration by positive ions. The starting resistance is applied in form of a thin strip or streak 10 of high resistance, and is preferably covered with a protecting insulating coating. It connects the anode with the casing 55 and the gauze 51. Starting is made particularly easy in that the end plate 1| of the cathode has a small distance from the gauze, so that at the starting electrons are shot immediately into the space 12, which becomes later the dark space.

Figs. 16-18 show an electron beam source for alternating current. This is a twin construction of two electron beam sources, which are emitting in turn in successive half-cycles. All inner discharge spaces are enclosed in a metallic casing 13, with a central partition 14. The plasmaiilter is 15. 'I'he virtual cathodes are formed b y the gauzes 16 and 11. The lower end of the casing is closed by the bottom plate 18, which is backed by a plate of insulating material 19. The two cathodes 80, 8| are introduced through holes of this plate. Their bare parts 82, 83 outside this plate are serving as anodes. These I have found that this is are connected with the leads 84, 85, which are preferably coated with insulating materials. The heating filaments 86, 81 are connected in series with each other and in parallel with the main current. Their central point 88 is used for energizing the starting electrodes. The starting devices are shown in Fig. 17. 89 is a sleeve on a high resistance, which divides it into two sections, 90 and 9|. 89 is connected with the central point of the heating laments. The outer sleeves 92 and 93 are connected with the inner casing 94 and with the outer casing 95. This outer or protecting casing has a higher potential during the starting process and makes it easier for the discharge to find the hidden anodes. During normal operation 95 assumes a potential which is only little less than the potential of the momentary anode and therefore it can not be disintegrated by ions. In Fig. 18 96l is the inner casing, 91 the central partition, 98 the protecting casing and 99, the two cathodes.

According to the invention not only hot cathodes, but also pool cathodes e. g. of mercury or alkali can be used. Fig. 19 shows a dark discharge amplier with a liquid cathode. |0| is the discharge vessel, into which a cup |02, which may be made, e. g., of ferrochrome, is sealed. This contains the cathode metal |03 and the funnel |04, which prevents the cathode spot from touching the glass. is the plasmaiilter, |06 the metal gauze. The carrying structure of |05 is connected with the conducting casing |01, which ts well into the glass envelope and has a lead |08. |09 is a grid, serving as ion-collector, which has a special lead ||0. |I| is the anode, sealed on its edge into the glass envelope. For starting the discharge, the casing |01 is connected with the anode across the resistance ||2. The cathode spot is started by any of the methods used for striking pool discharge devices, e. g., by high frequency.

In the dark discharge ampliers as described the emitted light intensity is controlled simultaneously with the current. Such devices can be therefore used advantageously for signaling, for talking nlm and for television purposes.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electric discharge device, with an envelope containing rareed gaseous substances, comprising at least one electron beam source with an inner discharge space corresponding with the outer discharge space only through openings of a. body or plasrnalter, which consists on its surface exposed to the outer discharge space and to the electron beams of insulating material, with a source of slow electrons at a distance of several mean free paths of molecules from the plasmafilter, the area of the electron source and its emissivity being so large as to supply the operating current up to an averaged current density in the openings of the plasmalter of at least 0.25 ampere/cin2 at the minimum ionic supply, which is a proportion of the electron current emitted expressed by the square foot of the ratio of electronic and ionic masses, the openings of the lter being so restricted as to preclude the formation of an ionizing discharge in the inner space.

2. An electric discharge device, with an envelope containing rarefled gaseous substances, comprising at least one electron beam source with an inner discharge space corresponding with the outer discharge space only through the openings of a body or plasmalter, which consists on its surface exposed to the outer discharge space and to the electron beams oi.' insulating material and on its inner surface of conducting material, connected with a hot cathode of the indirectly heated type at a distance of several mean free paths of molecules from the filter, said cathode having an area sufficiently large for emitting the operating current up to an averaged'current density in the openings of the plasmafilter of at least 0.25 ampere/cin2 at zero field, and the openings of the plasmafilter being so restricted as to preclude the formation of an ionizing discharge in the inner,

space.

3. An electric discharge device, with an envelope containing rarefied gaseous substances, comprising at least one electron beam source with an inner discharge space corresponding with the outer discharge space only through the openings of a body or plasmafilter, which consists on its surface exposed to the outer discharge space and to the electron beams of insulating material on and its inner surface of conducting material, connected with a perforated structure of conducting material with many fine openings, said structure having a distance of several mean free paths of molecules from the plasmafllter and separating the inner discharge space from a space containing a cathode, the openings both of the filter and of the perforated conducting structure being so restricted as to preclude the formation of an ionizing discharge in the inner space up to an operating current of such intensity as will produce an averaged current density of at least 0.25 ampere/cm2 in the openings of the plasmafilter.

4. An electric discharge device, with an envelope containing rarefled gaseous substances, comprising at least one electron beam source with an inner discharge space corresponding with the outer discharge space only through the openings of a body or plasmafilter, which consists on its surface exposed to the outer discharge space and to the electron beams of insulating material, said inner space containing parts with surfaces negative with respect to the potential of the inner discharge space, said surfaces restricting the passage of ions between the plasmafilter and the electron source, said electron source having an area and an emissivity sufficiently large for supplying the operating current up to an averaged current density in the openings of the plasmafilter of at least 0.25 ampere/cm2 at the minimum ionic supply', the openings of the plasmafilter being so restricted as to preclude the formation of an ionizing discharge in the inner space.

5. An electric discharge device, with an envelope containing rarefied gaseous substances, comprising at least one electron beam source with an inner discharge space corresponding with the outer discharge space only through openings of a body or plasmafllter, which consists on its surface exposed to the outer discharge space and to the electron beams of' insulating material, the inner discharge space containing a source of slow electrons and a gridlike structure made of fine wire, with a potential negative with respect to the potential of the inner discharge space and restricting the passage of ions from the filter to the electron source, said electron source having an area and an emissivity suficiently large for supplying the operating current up to an averaged current density in the openings of the filter of at least 0.25 ampere/cm2 at the minimum ionic supply, the openings of the plasmafilter being so restricted as to preclude the formation of an ionizing discharge in the inner space.

6. An electric discharge device, with an envelope containing rareiied gaseous substances, comprising at least one electron beam source with an inner discharge space corresponding with the outer discharge space only through openings of a body or plasmafilter, which consists on its surface exposed to the outer discharge space and to the electron beams of insulating material, the inner discharge space containing an electron source of an area and emissivity sufficiently large for supplying an operating current with an averaged current density in the openings of the filter of at least 0.25 ampere/cm2, said source of slow electrons being situated behind a shield in such a position as to be protected against such ions which penetrate the openings of the filter and do not suffer at least one collision in the inner space, the openings of the plasmafilter being so restricted as to preclude the formation of anionizing discharge in the inner space.

7. An electric discharge device, with an envelope containing rarefied gaseous substances, comprising at least one electron beam source with an inner discharge space corresponding with the outer discharge space only through openings of a body or plasmafilter, which consists on its surface exposed to the outer discharge space and to the electron beams of insulating material, with a source of slow electrons at a distance of several mean free paths of molecules from the filter, the area of the electron source and its emissivity being so large as to supply the operating current up to an averaged current density in the openings of the filter 0f at least 0.25 ampere/cmz at the minimum ionic supply, and the openings of the plasmafilter being so restricted as to preclude the formation of an ionizing discharge in the inner space, means being provided for creating a magnetic field with a component parallel to the surface of the source of slow electrons.

8. An electric discharge device for producing light, with an envelope containing rarefied gases, said envelope having a. tubular part with an anode at one end and a wider part containing an electron beam source with an inner discharge space, which corresponds with the outer discharge space only through the openings of a body or plasmafilter, which consists on its surface exposed to the outer space and to the electron beams of insulating material, with a source of slow electrons at a distance of several mean free paths of molecules from the filter, the area of the electron source and its emissivity being so large as to supply the operating current up to a current density averaged in the openings of the plasmafilter of at least 0.25 ampere/cm2 at the minimum ionic supply, and the openings of the plasmafilter being so restricted as to preclude the formation of an ionizing discharge in the inner space.

9. An electricdischarge device for producing light, with an envelope containing rareed gaseous substances, said envelope having a tubular part connecting two wider parts, which contain each an electron beam source having an inner discharge space, which corresponds with the outer discharge space only through the openings of a body or plasmaiilter, which consists on its surface exposed to the outer space and to the electron beams of insulating material, with a source of slow electrons at a distance of' several mean free paths of molecules from the filter, the area of the electron source and its emissivity being so large as to supply the operating current up to a current density averaged in the openings of the being so restricted as to preclude the formation of an ionizing discharge in the inner space.

10. Method of operating an electric discharge device with an envelope containing rareed gaseous substances, comprising at least one electron beam source, with an inner discharge space corresponding with the outer discharge space only through openings of a body or plasmaillter, which consists on its surface exposed to the outer space and to the electron beams of insulating material, with a hot cathode of the indirectly heated type at a distance of several mean free paths of molecules from the plasmalter, the openings of which are so restricted as to preclude the formation of an ionizing discharge in the inner space, and heating said hot cathode up to such a temperature that at an operating current of atleast 0.25 ampere/cm2 averaged current density in the openings of the plasmaillter, a further rise of temperature does no more produce a rise of current.

11. Method of controlling the volt-ampere characteristics of an electric discharge device with an envelope containing rareiied gaseous sub- Patent No. 2,15 6, 292.

stances, comprising at least one electron beam source with an inner discharge space, which corresponds with the outer space only through the openings oi a body or plasmalter which consists on its surface exposed to the outer space and to the electron beams of insulating material, with a. source of slow electrons at a distance of several mean free paths of molecules from the illter and a gridlike structure made of fine Wire between illter and electron source, said electron source having an area sufficiently large and being operated with an emissivity sulcient for supplying an operating current to produce an averaged current density of at least 0.25 ampere/cmz in the openings of the plasmalter at the minimum ionic supply, by means of impressing negative potentials on said gridlike structure and by this, intercepting part of the ions moving from the openings of the filter towards the electron source, the openings of the plasmaiilter being so restricted that no appreciable number of ions is produced within the inner space itself.

DNEs GABOR.

llovember "8, 1958 DE'NEs cABoR.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Pagel, second column, 'line 55, for the reference numeral "'ll read -10'; page2, first column, line "(5, for "irons" read --ions--g and second column, line 10-11, for "iron-supplying" read --ion-supplyingjn; line 16, in the formula,

for "d s" read -.d; `s--g-page'l, second column, line 65, claim 1, for "foot" read rootA-; page 6, first co1umn,-1i ne 20, claim 5, for "on" read and; line 21, same claim, for "'and" lrend -on; and that the sai-d Letters Patent should berend with this correction therein that the same may conform to the record of 'the case inthe Patent Office. '4

Signedand sealed this llth'day of June, A. D. l9li0.

. A l Henry Van` Arsd'ale `(Seal) Acting Commissioner of' Patents. 

